This invention relates to a process and a filter for filtering solids from liquids, particularly when the solids and the liquids have substantially different specific gravities.
Processes and filters for filtering liquids are well known.
Problems occur with conventional techniques when the specific gravities of the substances to be separated are too disparate. When liquids having a light specific gravity are filtered with the use of the custumary filter aids, it is again and again observed that the filter cake which becomes deposited on the filter forms ridge-like areas at the edges of the filter element. The ridge-like areas are the result of irregular deposition of solids forming the filter cake. Because of the irregular deposition of the filter cake, the filter aid is exhausted too early. This premature exhauston is particularly unfortunate because the filter aid typically accounts for a high portion of the expense of filtering. Another disadvantage of the irregular deposition is the consequent tendency of the filter element to become unbalanced because of its non-uniform load. Still another significant disadvantage becomes apparent when there is an attempt to terminate filtration when filter cake attains a predetermined thickness. The ridges tend to prematurely terminate filtration since only the ridges are at the predetermined thickness.
The same phenomenon of ridges at the edges of the filter element is observed with the deposition of relatively heavy ion exchange resin powder in aqueous solutions. This problem is particularly important in the treatment of condensates being returned to power stations.
Generally, this phenomenon arises when the density variation between the liquid and the solids (such as the filter aid and the material forming the filter cake) exceeds a certain value. This value is largely dependent on the nature of the liquid-solid system. The distribution of the sizes of the granules of the solids and the sedimentation rate also play a crucial role in the filtration.